1. Field of the Invention
The invention relates to an information storage medium, a managed product and a management system that use IC tags for wireless communication.
2. Description of the Related Art
Wireless communication technology is applied not only to the field of information and communication but also to the field of logistics management, and IC tags for wireless communication (hereinafter referred to simply as “IC tags” or “tags”), which are widely known as products that support the RFID (Radio Frequency Identification) technology, have a wide variety of applications including use in logistics management and as inexpensive information storage mediums. For this reason, wireless communication equipment will be used in a variety of application environments.
IC tags can be classified into passive tags that do not have their own power source and active tags having a power source. The passive tags, which are low cost and versatile, receive a radio wave from a reader and activate the tag circuit by using this energy, thereby enabling wireless communication.
IC tags each include an IC chip for storing data, such as identification number, and an antenna for transmission and reception of radio waves, and have great advantages including a thin and lightweight design and a wireless communication capability. The chip includes a memory region, and therefore information management is possible using information stored in the memory region, and the IC tag serves as an information storage medium. IC tags are also called “tags”, “wireless IC tags”, “RFID tags” or “RF tags”. In the invention, a combination of a commercially available IC tag and a sheet body of the invention is referred to as a “tag”. In order to distinguish this from others, such a combination of an IC tag and a sheet body will be referred to as a “tag according to the invention”.
In order to make full use of the advantages, it is considered preferable that there is no limitation on the tag application position and that IC tags are configured to be capable of communication regardless of the application position.
IC tags, however, are designed for use in free space, and in the case of using a radio wave in the high frequency band, the ultra-high frequency band or the microwave band, general-purpose tags performs transmission and reception of radio wave communication, using a so-called dipole antenna, and thus if there is a metal or the like in the vicinity of the antenna, the communication characteristics of the antenna are degraded and a possible communication distance is reduced.
When a wireless radio wave impinges on a metal surface, reflection occurs with a phase shift by λ/2. In other words, incident and reflected waves cancel each other in the vicinity of the metal. In the vicinity of the metal, the composite electric field becomes close to 0, and thus the supplied electric field energy will be reduced significantly as compared to that in free space.
Also, if there is a conductive material such as a metal in the vicinity of the antenna, when a resonance current flows through the antenna, a current reverse to the resonance current is induced on the metal side, and the input impedance of the antenna is reduced greatly due to the induced current, failing to match the impedance of the IC chip designed for use in free space, and reducing the possible communication distance. Also, if the same magnitude of current flows in parallel in a reverse direction in the vicinity of the antenna, the orientation of magnetic fields generated around the currents is reversed, causing the magnetic fields to cancel each other, and as a result, the radio waves do not travel far and the possible communication distance is reduced.
Other than metals, materials such as paper, glass, resins and liquids can also degrade the communication characteristics of IC tags.
In the case of these materials, the resonance frequency of the antenna changes due to the dielectric constant and the magnetic permeability of these materials, causing a shift between the frequency of radio wave used by the communication receiver and the resonance frequency of the antenna, and thus the possible communication distance is reduced.
Japanese Unexamined Patent Publication JP-A 2009-134709 discloses a wireless communication improving sheet body with which the possible communication distance can be improved even in the vicinity of a communication disturbing member such as a metal.
With the wireless communication improving sheet body disclosed in JP-A 2009-134709, the possible communication distance can be extended by using it when, for example, an IC tag is affixed to a metal surface.
However, use of the wireless communication improving sheet body inevitably creates, in the IC tag, a possible communication position and an impossible communication position. When an attempt is made to read the IC tag from the back side of the wireless communication improving sheet body, or in other words, an opposite side of an arrangement surface of the IC tag, by using the reader, reading is almost impossible.
When an attempt is made to apply such IC tags to management of members, for example, because the IC tags include an unreadable region, information cannot be read from the IC tags unless all of the IC tags face the reader, and consequently member management will be substantially impossible. From the viewpoint of member management, there is a demand to make the unreadable region as small as possible.